1. Field of the Invention
The present invention relates to communications cables. More particularly, the present invention relates to filler rods for stranded optical fiber cables made from polypropylene based materials.
2. Description of the Prior Art
Optical fiber cables have been in use in the communications industry for a number of years to transmit information at very high rates over long distances. Optical fiber cables come in a variety of configurations. The configurations include: cables with a centrally located single buffer tube containing one or more optical fibers; cables with a plurality of buffer tubes stranded in a helical or alternating helical arrangement about a central strength member; and cables with slotted cores in which a plurality of optical fibers reside.
In stranded optical fiber cables, each of the buffer tubes typically contains up to 12 optical fibers or more, or in some cases ribbons. The buffer tubes are usually stranded in an alternating helical or S-Z configuration. For example, a 72 count stranded optical fiber cable has a core containing six buffer tubes arranged about a central strength member and each of the six buffer tubes contains 12 optical fibers. A protective jacket is usually extruded over the core. Other strength members such as aramid yarns, ripcords and water blocking members can be included in the cable.
In some cases, it is desirable to have a lower fiber count in a standard configuration cable having the same symmetrical concentricity. For example, it may be desirable to have a 36 count stranded optical fiber cable which is substantially the same in terms of concentric structure to a 72 count cable except for the reduced number of fibers. When a reduced count cable is made, normally one or more of the buffer tubes are replaced by filler rods. The filler rods fill the space that would normally be occupied by a buffer tube containing optical fibers so as to keep the structure of the cable intact. For example, cables having 12, 18, 24, or 36 fiber counts can use the same basic cable structure and therefore have the same overall diameter despite different fiber counts. The filler rods also permit the manufacturing lines for making such optical fiber cables to remain substantially unchanged, whether making a low or high count optical fiber cable.
Prior to the present invention described below, filler rods were typically made with solid or with foamed high density polyethylene (HDPE). When foamed HDPE was used, it was foamed to approximately 90% of the unfoamed HDPE density. Foamed HDPE was an attractive filler rod material mainly due to low cost. However, some problems experienced with filler rods made from such material in some environments in which cables containing such rods includes excessive post extrusion shrinkage and reduced crush resistance, especially in low count cables with many filler rods. Also, processing cables with reduced thermal barriers between the jacket and core have created a demand for filler rods made with a material which has better dimensional stability at higher temperatures. In some cases, outer jackets of cables are made with materials having a higher melting point than the HDPE material. When such materials are extruded over the HDPE filler rods, some of the HDPE melts causing it to become intermingled with the molten outer jacket material. When this happens, the filler rods can stick to the outer jacket, a phenomenon which is undesirable because it decreases the ease of cable and fiber accessibility.
It is generally understood by those skilled in the art that reliability of cable components, such as filler rods, over a long time period and under extreme environmental conditions is very important. For example, it is desirable that filler rods for optical fiber cables undergo a minimum amount of shrinkage during the cable's lifetime. It is also desirable to provide filler rods designed to have physical properties which are compatible with the physical properties of the material currently used for the buffer tubes in such cables, such as that found in U.S. Pat. No. 5,574,816, so that both the buffer tubes and filler rods respond in a similar manner to changing environmental conditions.